The proposed objectives are: 1. To define the relevant antigens (VEA, TSSA or others) involved in tumor recognition in chickens and to determine the relative contributions of the cellular and humoral limbs of the chicken immune system in tumor growth and rejection in vivo and as assessed by in vitro correlates of immunity. 2. To determine if T cells from RSV and ALV immunized chickens recognize TSSA and VEA of other heterologous ATV subgroups via group-specific determinants. 3. To further investigate the immune competence of birds congenitally infected with ALV. To determine if leukocytes (T and B cells) from birds congenitally infected with ALV fail to recognize VEA or TSSA of homologous and heterologous subgroups and if leukocytes deliberately infected in vitro, as well as in vivo with ALV can recognize TSSA or VEA. 4. To define the importance of antibody dependent cellular cytotoxicity (ADCC) in birds bearing RSV-induced tumors and in birds infected with ALV and to correlate ADCC with other parameters of the immune response and the outcome of ATV-induced neoplasia. 5. Characterization of the specificity and avidity of normal and ALV congenitally-infected chickens' humoral response to whole virus and to purified virion structural antigens.